The present invention relates generally to coating technology, and more particularly to a process for sputtering boron, copper, or titanium nitride with low frequency alternating current. The process has a wide range of applications. It has a specific application for coating tokamak wails in fusion reactors with boron. In addition, the invention has general application for coating any mechanical device subject to wear or corrosion with various materials.
Boron coating, or "boronization" of tokamaks, with either chemical vapor deposition of boron from boron containing compounds, or boronization from evaporation of solid targets of boron/carbon mixtures, has produced beneficial results. Boron has properties that are advantageous to improved reactor performance. As a material that comes in contact with the plasma edge, it has been found to reduce the influx of some materials, the recycling of hydrogenic species, the oxygen and oxygenated species, and even lower the loop voltage. The presence of boron also helps maintain the low atomic number needed for a tokamak coating. Additionally, the use of boron coatings may have widespread applicability in vacuum systems for electrical and/or corrosion resistance in contained environments with controlled atmospheres under non-vacuum conditions.
On a much broader scale than the boron coating of tokamaks, billions of dollars are spent annually to replace mechanical devices that are subjected to wear caused by abrasion, erosion, or corrosion during the course of normal service. Many of these components fail prematurely due to excessive wear in inert and corrosive service environments, and could be made to last longer by the use of harder and corrosive resistant materials. However, the cost of doing so is often prohibitively high.
Titanium nitride (TiN) is one compound that is used to deposit a hard durable coating on various materials to provide wear resistance or corrosion resistance or other similar properties. Generally these coatings have been produced on metal articles by chemical vapor deposition (CVD) or similar processes. CVD is used to deposit TiN to produce wear-resistant, nonabrasive surfaces on plain carbon steel articles. It is also used on external ornamental metal parts such as wrist watch cases, bands or bezels, to obtain a gold-colored surface having corrosion and wear resistance properties. TiN can also be used to place a hard durable coating on soft metals such as aluminum.
Coating with materials like TiN is generally done commercially by using ion plating techniques. Chemical vapor deposition of coatings requires high capital investment, high processing costs and is limited to very thin coatings and small parts. Of the thermal spray deposition methods which can be used to coat parts of unlimited size with coatings of unlimited thickness, flame spraying often yields a porous coating with oxide inclusions. There are a number of additional methods available for surface hardening or depositing corrosion and wear resistant materials on industrial parts. Among these methods are D.C. sources, electron beams, R.F. plasmas, diffusion treatments, nitrating and carburizing of ferrous based materials, electroplating, and sputtering.
A coating technique has been developed that resembles ion plating deposition but uses simple components. The technique reduces the cost by about two orders of magnitude less than currently used methods. This technique, using low frequency alternating current is also employed to produce adherent and thick coatings on metal without the need for nickel or copper striking.
Accordingly, it is an object of the present invention to provide an a.c. coating technique that uses simple components and reduces the cost compared to currently used coating methods.
It is another object of the present invention to produce adherent and thick coatings (by electroplating) of copper on stainless steel without the need for nickel and copper striking.
A further object of the present invention is to utilize the properties of an ionized gas to self-rectify low frequency (60 Hz) alternating current for sputtering.